A gas turbine is constituted by a compressor, a combustor, and a turbine. The air taken in from an air inlet is compressed by the compressor, and is supplied to the combustor as a high-temperature and, high-pressure compressed air. In the combustor the compressed air and fuel are mixed and combusted, and the result is supplied to the turbine as a high-temperature and high-pressure combustion gas. In the turbine, a plurality of stator vanes and turbine blades are alternately disposed within a casing, the turbine blades are rotationally driven by the combustion gas supplied to an exhaust passage, and the rotational driving is recovered as electric power by a generator coupled with a rotor. The combustion gas which has driven the turbine is converted into hydrostatic pressure by a diffuser, and is emitted to the atmosphere.
In the gas turbine configured in this way, there is a possibility that the temperature of the combustion gas which acts on the plurality of stator vanes and turbine blades reaches 1500° C., and the stator vanes and the turbine blades are heated and damaged. Therefore, in the stator vanes and the turbine blades, a cooling flow passage is provided in an air foil, a blade wall is cooled by a cooling medium, such as cooling air received from the outside, and when the cooling medium is made to flow into the combustion gas from cooling holes provided in the blade wall, the surface of the blade is cooled by film cooling, etc.
Meanwhile, between a blade tip (apex) of each turbine blade which is rotationally driven, and a ring segment constituting the portion of the casing, a predetermined gap is provided so that both the blade tip and the ring segment do not interfere with each other. However, if the gap is too large, since a portion of the combustion gas flows over the blade tip and flows away to the downstream, energy loss occurs, which reduces the thermal efficiency of the gas turbine. In order to suppress the leak of the combustion gas from this gap, the blade tip of the turbine blade is provided with a squealer (also referred to as a thinning) which functions as damming, and the gap between the top surface of the squealer and the ring segment is made as small as possible to prevent a decrease in the thermal efficiency of the gas turbine.
An example of such a turbine blade is shown in FIGS. 5A and 5B.
A turbine blade 50 shown in FIG. 5A is erected on a platform 11 embedded in a rotating rotor disc (not shown) via a blade root portion 16, and a rotor (not shown) and the rotor disc (not shown) rotate integrally. When the section of the turbine blade 50 is seen from the radial direction of the blade, a pressurized-surface-side blade wall 18 is concavely formed from a leading edge to a trailing edge on the upstream of the blade in its rotational direction R, and a suction-surface-side blade wall 19 is convexly formed from the leading edge to the trailing edge end on the downstream of the blade in its rotational direction R. A blade tip 15 of the turbine blade 50 is blocked by a top plate 17. On the top plate 17, a squealer 23 is provided in the shape of a belt from the leading edge side to the trailing edge side along the suction-surface-side blade wall 19 in the peripheral direction of the turbine blade 50, and protrudes radially outward from the blade. In this configuration, a portion of a combustion gas FG which has come into contact with the blade surface from the turbine blade 50 on the side of the pressurized-surface-side blade wall 20 flows along the top plate 17 of the blade tip 15, flows over the squealer 23, and flows to a downstream exhaust passage.
As shown in FIG. 5B, in order to cool the top plate 17 and the squealer 23, the blade tip 15 of the turbine blade 50 is provided with cooling holes 28a and 28b through which a portion of the cooling medium CA which flows through the cooling flow passage 26 within the air foil 12 is blown off into the combustion gas. Additionally, although a portion of the combustion gas FG flows through the gap C between the ring segment 60 and the top surface 23a of the squealer 23, this gap flow causes the energy loss of the turbine, and causes a decrease in the thermal efficiency of the gas turbine. Accordingly, it is contrived to make the gap C as small as possible. Therefore, depending on the operating conditions of the gas turbine, the top surface 23a of the squealer 23 and the lower surface of the ring segment 60 rotate while being brought into contact with each other by the rotation of the turbine blade 50.
Additionally, in order to protect the blade surface directly exposed to the high-temperature combustion gas, a heat-resistant coating (also referred to as TBC) 24 is applied on outside surfaces, such as the top plate 17 of the blade tip 15, the suction-surface-side blade wall 19, the pressurized-surface-side blade wall 20, and a side wall 23d of the squealer, thereby interrupting the heat from the high-temperature combustion gas in order to prevent the damage of the blade surface. In this regard, as described above, since the gap C between the top surface 23a of the squealer 23 and the ring segment 60 is adjusted so as to be as small as possible, it is difficult to apply a heat-resistant coating on the top surface 23a of the squealer 23, and the base material of an air foil is exposed to the combustion gas. Therefore, the top surface 23a of the squealer is protected from the high-temperature combustion gas by the convection cooling of the cooling medium CA which flows through the cooling holes 28b. 
Examples of turbine blades in which a squealer is provided at the whole periphery of a blade wall are disclosed in Patent Documents 1 to 3.    [Patent Document 1] Japanese Patent Unexamined Publication, First Publication No. 2004-169694    [Patent Document 2] Japanese Patent Unexamined Publication, First Publication No. 2001-107702    [Patent Document 3] Japanese Patent Unexamined Publication, First Publication No. 2008-051094